In an internal combustion engine of carburetor type, the temperature of the fuel in the float chamber of the carburetor located near the engine body is apt to be increased just after the engine is stopped or when the engine is operating in the idle condition. This is because, since the cooling fan is not operated or is insufficiently operated when the engine is operating under such conditions, the carburetor located near the engine body is subjected to the high temperature of the engine body. Due to the increase of the temperature of the fuel just after the engine is stopped, the fuel is evaporated, and is lost to the atmosphere, causing a bad fuel consumption efficiency as well as public pollution which is now legally restricted. Further, if the fuel is evaporated during the engine idle condition, it is impossible to obtain a predetermined air-fuel ratio, and this causes rough idle operation of the engine.
For preventing the increase of the temperature of the fuel in the carburetor due to the high temperature of the engine body, a device has already been proposed that has a supplementary cooling fan facing the carburetor. The supplementary fan is operated when the air temperature in the engine room is increased to a predetermined temperature in order to cool the carburetor body whereby an increase of the temperature of the carburetor can be prevented. However, this type of device suffers from such drawbacks that a special design is necessary for the vehicle body, because of the limited space of the engine room, and that the supplementary fan requires a relatively large power consumption, as high as 45-80W, so that it is necessary to increase the capacity of the battery of the engine.